It looks like it's possible that Thomas Jefferson may have belonged to Y
haplogroup O. The list below compares the haplotype of Jefferson to six
examples of haplogroup O given in Butler's 20 STR multiplex paper.
Jefferson's haplotype comes fairly close to a couple of them.

DYS: 019 388 389i 389ii 390 391 392 393

Jeff 15 12 12 27 24 10 15 13

O1 15 12 12 27 23 10 14 13

O1 15 12 12 28 23 10 14 13

O2a 15 12 13 30 25 11 13 14

O3c 15 12 13 28 24 10 14 12

O3e 15 12 12 28 23 10 12 12

O3e 15 12 12 28 23 10 12 13

A recent study by Paracchini et al of more than 1000 Californians and
Hawaiians saw haplogroup O among only about 1% of the men (except for those
of Japanese descent). The results of that study are summarized here: